Weather and Desert Locusts - WMO-No. 1175 - Humanitarian Library
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WEATHER CLIMATE WATER
Weather and
Desert Locusts
WMO-No. 1175
WMO-No. 1175 © World Meteorological Organization and Food and Agriculture Organization of the United Nations, 2016 The right of publication in print, electronic and any other form and in any language is reserved by WMO and FAO. Reproduction and dissemination of material in this information product for educational or other non-commercial purposes are authorized without any prior written permission from the copyright holders, provided the source is fully acknowledged and that neither WMO nor FAO endorsement of users’ views, products or services is implied in any way. Reproduction of material in this information product for resale or other commercial purposes is prohibited without written permission of the copyright holders. Editorial correspondence and requests to publish, reproduce or translate this publication in part or in whole should be addressed to: Chairperson, Publications Board World Meteorological Organization (WMO) 7 bis, avenue de la Paix Tel.: +41 (0) 22 730 84 03 P.O. Box 2300 Fax: +41 (0) 22 730 81 71 CH-1211 Geneva 2, Switzerland E-mail: publications@wmo.int ISBN 978-92-63-11175-3 (WMO) ISBN 978-92-5-109426-6 (FAO) Cover illustration: Adobe Stock NOTE The designations employed and the presentation of material in this publication do not imply the expression of any opinion whatsoever on the part of WMO or FAO concerning the legal status of any country, territory, city or area, or of its authorities, or concerning the delimitation of its frontiers or boundaries. The mention of specific companies or products of manufacturers, whether or not these have been patented, does not imply that they are endorsed or recommended by WMO or FAO in preference to others of a similar nature which are not mentioned or advertised. In this publication, the findings, interpretations and conclusions expressed by named authors are those of the authors alone and do not necessarily reflect those of WMO, FAO or FAO Members.
Weather and Desert Locusts WMO-No. 1175
CONTENTS Page
Foreword�������������������������������������������������������������������������������������������������������������������������������������������������� iv
Acknowledgements�������������������������������������������������������������������������������������������������������������������������������� vi
Introduction�������������������������������������������������������������������������������������������������������������������������������������������� vi
Desert Locusts ����������������������������������������������������������������������������������������������������������������������������������������� 1
Overview������������������������������������������������������������������������������������������������������������������������������������������ 1
Life cycle������������������������������������������������������������������������������������������������������������������������������������������ 3
Affected areas �������������������������������������������������������������������������������������������������������������������������������� 7
Plagues and upsurges�������������������������������������������������������������������������������������������������������������������� 8
Weather����������������������������������������������������������������������������������������������������������������������������������������������������� 9
Weather and Desert Locust biology �������������������������������������������������������������������������������������������� 9
Weather and locust control operations�������������������������������������������������������������������������������������� 11
Weather and Desert Locust plagues and upsurges������������������������������������������������������������������ 12
Importance of weather information and conditions for Desert Locust control�������������������� 15
Interpreting weather maps �������������������������������������������������������������������������������������������������������� 16
Interpreting rainfall-analysis maps �������������������������������������������������������������������������������������������� 22
Satellites and models ������������������������������������������������������������������������������������������������������������������ 23
The impacts of climate change �������������������������������������������������������������������������������������������������� 27
Organizational considerations �����������������������������������������������������������������������������������������������������������30
National Meteorological and Hydrological Services and National Locust
Control Centres ���������������������������������������������������������������������������������������������������������������������������� 30
The World Meteorological Organization and the Food and Agriculture
Organization �������������������������������������������������������������������������������������������������������������������������������� 34
Glossary���������������������������������������������������������������������������������������������������������������������������������������������������36
References�����������������������������������������������������������������������������������������������������������������������������������������������38
iv WEATHER AND DESERT LOCUSTS FOREWORD Desert Locust plagues can be an important contributing factor to famines and a threat to food security in many regions of the world. The Desert Locust plague of 1986–1989 and subsequent upsurges during the past two decades demonstrate the continuing capacity of this historic pest to threaten agriculture and livelihoods over large parts of Africa, the Near East and South- West Asia. In 2004–2005, a major upsurge caused significant crop losses in West Africa, with a negative impact on food security in the region. These events emphasize the need to strengthen and maintain a permanent system of well-organized surveys in areas that have recently received rains or been flooded, supported by a control capability to treat Desert Locust hopper bands and adult swarms efficiently in an environmentally safe and cost-effective manner. The events of the 1986–1989 plague showed that, in many instances, the existing strategy of preventive control did not function well, for reasons which included the inexperience of the field survey teams and campaign organizers, lack of understanding of ultra-low volume spraying, insufficient or inappropriate resources and the inaccessibility of some important breeding areas. These reasons were compounded by the general tendency to allow survey and control capacity in locust-affected countries to deteriorate during locust recession periods. Given the certainty that there will be future Desert Locust upsurges, the Food and Agriculture Organization of the United Nations (FAO) produced a series of guidelines and standard operating procedures primarily for use by national and international organizations and institutions involved in Desert Locust surveys and control. The infrequency and brevity of locust plagues is welcome. However, long locust recession periods may be a source of problems. When a new campaign is needed, few national locust staff and even fewer pilots are likely to have campaign experience. Manpower and equipment resources on hand may have been diverted and, hence, are not available, or are insufficient, to run a campaign. Morale is likely to be low after a decade or more with only rare seasonal activity of any significance. Inexperienced staff from outside the national locust unit are likely to be deployed and every available sprayer used, whether suitable or not. Furthermore, lack of funds, bureaucratic rigidity and poor security in locust-affected countries can easily hamper a timely and effective response. When there is an upsurge, pesticide and aircraft are often supplied far in excess of what can be deployed currently, even by a well-trained, well-organized unit. Furthermore, while the possible effects of climate change on Desert Locust are still under study, depending on the region, policymakers may have to prepare for longer locust seasons. This calls for strengthening international collaboration to better study the behavioural patterns of Desert Locusts in relation to changing meteorological and climatic conditions and to adapt control and preparedness plans. FAO and the World Meteorological Organization (WMO) collaboration on Desert Locusts started in 1951, when the WMO Technical Assistance Mission for Desert Locust Control was established. Major movements of swarms were hypothesized to take place downwind, towards and within zones of convergent surface windflow. In 1981, meteorologists participated for the first time in the meeting of the FAO Commission for Controlling the Desert Locust in North-West Africa that was held in Algiers, 14–19 March 1981 (WMO, 1992). As the relationship between meteorological conditions and locust activity had been known for many years, the basis for cooperation with National Meteorological and Hydrological Services (NMHSs) was thereby established. Meteorologists from WMO Members have since been involved in national Desert Locust programmes and in joint action by WMO and FAO. FAO is the lead agency in Desert Locust monitoring and control and runs the Desert Locust Information Service (DLIS). All locust-affected countries transmit locust and environmental data, as well as survey and control results, to DLIS for analysis, in conjunction with weather and habitat data and satellite imagery, in order to assess the current locust situation, provide forecasts of up to six weeks, and issue early warnings. FAO routinely prepares monthly locust bulletins and periodic updates forecasting the scale, location and timing of locust migration and breeding on a
WEATHER AND DESERT LOCUSTS v country-by-country basis. This information constitutes the early warning system operated by DLIS to alert countries and donors about the development of plagues. DLIS disseminates information by e-mail, the Locust Watch website (http://www.fao.org/ag/locusts) and social media. In cooperation with affected countries, FAO undertakes field assessment missions and coordinates survey and control operations, as well as assistance during locust emergencies. To address the deterioration of survey and control capacity during recession periods, FAO has given high priority to a special programme, the Emergency Prevention System for Transboundary Animal and Plant Pests and Diseases to strengthen national capacities. As the authoritative voice on weather, water and climate of the United Nations system, WMO provides valuable assistance to FAO to ensure that WMO Members and their NMHSs provide near-real-time meteorological data and forecasts for locust-affected countries. WMO also maintains a Meteorological Information for Locust Control web page on the site of the World AgroMeteorological Information Service (WAMIS). WAMIS is a centralized web server that disseminates agrometeorological products issued by WMO Members (http://www.wamis.org/ locust/index.php). The complex and serious nature of Desert Locust plagues demands that countries that are invaded and those that are threatened by them to work together across borders to find the best form of locust control. Desert Locust control is indeed an international responsibility, because locusts breed and move over wide areas so that events in one country rapidly affect events in others. Together, WMO and FAO have been helping to improve coordination and planning for potential locust outbreaks and control actions. Petteri Taalas José Graziano da Silva Secretary-General, WMO Director-General, FAO
vi WEATHER AND DESERT LOCUSTS ACKNOWLEDGEMENTS FAO and WMO would like to thank Keith Cressman and Robert Stefanski for their work on developing this publication. They would also like to thank Emma Daniels for her assistance in editing the document. INTRODUCTION This publication is mainly intended to be used as a general reference guide for use by staff of National Locust Control Centres (NLCCs) and NMHSs of locust-affected countries. It may also be useful for anyone wanting to know more about Desert Locust and associated meteorological phenomena. The publication contains basic information on the biology and behaviour of Desert Locust, a history of locust events, weather factors that influence locust development and how to use weather information. The reader is encouraged to consult the following websites for more information: FAO Locust Watch http://www.fao.org/ag/locusts/en/info/info/index.html WMO Meteorological Information for Locust Control http://www.wamis.org/locust/index.php
WEATHER AND DESERT LOCUSTS 1
DESERT LOCUSTS
Overview
Locusts are members of the grasshopper family Acrididae, which includes most short-horned
grasshoppers. Locusts differ from grasshoppers because they have the ability to change their
behaviour and physiology, in particular their morphology (colour and shape), in response to
changes in density, when meteorological conditions are favourable. Adult locusts can form
swarms that may contain millions or billions of individuals that behave as a coherent unit
(Figure 1). The non-flying hopper (or nymphal) stage can form cohesive masses that are called
hopper bands.
Desert Locusts (Schistocerca gregaria) are always present somewhere in the deserts between
Mauritania and India. When numbers are low, they behave as individuals (solitarious phase);
when high, they behave as a single mass (gregarious phase). Colour and shape are an indication
of how they been behaving but may not be a reliable guide as to how they will behave in the
future.
When plentiful rain falls and annual green vegetation develops, Desert Locusts can increase
rapidly in number and, within a month or two, start to concentrate and become gregarious.
Unless checked, this can lead to the formation of small groups or bands of wingless hoppers and
small groups or swarms of winged adults. This is called an outbreak and usually occurs within an
area of about 5 000 km2 (100 km by 50 km).
An outbreak or contemporaneous outbreaks that are not controlled can evolve into an upsurge
if widespread or unusually heavy rain falls in adjacent areas, creating favourable breeding
conditions. An upsurge generally affects an entire region and occurs after several successive
seasons of breeding and further hopper-band and adult-swarm formation takes place.
(a) (b)
(c) (d)
Figure 1. Desert Locust (a) hopper, (b) hopper band, (c) adult and (d) swarm. Hoppers are the
wingless juvenile stage, while adults can fly and reproduce. Under optimal conditions, hoppers
can form bands and adults can form swarms.
2 WEATHER AND DESERT LOCUSTS
If an upsurge is not controlled and ecological conditions remain favourable for breeding, locust
populations continue to increase in number and size, and the majority of locusts behave as
gregarious bands or swarms, then a plague can develop. A major plague exists when two or
more regions are affected simultaneously.
Although outbreaks are common, only a few lead to upsurges. Similarly, few upsurges lead to
plagues. The last major plague was in 1986–1989 and the last major upsurge, or regional plague,
was in 2003–2005. Upsurges and plagues do not occur overnight; they take many months
to develop. During plagues, Desert Locusts may spread over an area of some 29 million km2,
extending over or into parts of some 60 countries.
The Desert Locust has the potential to damage the livelihoods of one tenth of the world’s
population. Recent increases in cultivated areas on the edges of deserts in northern Africa, the
Near East and South-West Asia make the Desert Locust a threat to the livelihood, income and
food source of local populations. Swarms are often tens of square kilometres in size. A swarm of
1 km2 eats the same amount of food in one day as 35 000 people. A swarm the size of Bamako
(Mali) or Niamey (Niger) can consume what half the population of either country would eat in a
single day.
The Desert Locust plague of 1986–1989, the subsequent upsurges in the 1990s and the regional
plague in 2003–2005, drew the world’s attention to the threat they pose to the food security of
the affected countries, especially in the developing world. This situation calls for an integrated
approach to understanding the conditions that lead to the locust build-up and their migration so
that effective solutions can be developed for controlling damage.
Locust control is complicated by several factors:
(a) The swarm is highly mobile, migrating from 50 to more than 100 km in a day;
(b) The total invasion period frequently occurs in a relatively brief time, sometimes as short as a
month but rarely longer than three months;
The Desert Locust cycle
Under optimal conditions The female locust lays
the Desert Locust changes her eggs directly into
both colour and behaviour the soil that hatch
as it becomes more after two weeks.
gregarious.
GREGAR
IO
SWARM U
S
Increasing
multiplication and
concentration
BAND
Adults mature and
SOLI The hopper moults
are ready to lay
eggs after at least
TARY 5-6 times and
becomes an adult in
3 weeks. about 4-6 weeks.
Figure 2. Desert Locust life cycle. The lifetime of a Desert Locust is about three months but this
can be extended to up to six months under cold conditions.WEATHER AND DESERT LOCUSTS 3
(c) The swarms are unevenly distributed in time, so that very large swarms may be available for
only a few days, followed by relatively long periods when none is present;
(d) Swarms are variable in size and can extend up to thousands of hectares;
(e) Campaign experience, funds and supplies are often lacking in locust-affected countries
because of the irregular occurrence of locust upsurges and plagues.
Life cycle
A Desert Locust lives about three to five months, although this is extremely variable and depends
mostly on weather and ecological conditions. The life cycle comprises three stages: egg, hopper
(nymph) and adult (Figure 2). Eggs hatch in about two weeks, depending on temperature (the
range is 10–65 days). Hoppers shed their skins five or six times, each time growing in size. This
process is called moulting and the stage between moults is referred to as an instar. Hoppers
develop over a period of about 30-40 days; adults mature in about three weeks to nine months
but more frequently from two to four months, depending on environmental conditions, mainly
temperature. If conditions are dry and cool, adults may remain immature for six months.
Adults do not moult and therefore do not grow in size but gradually increase in weight. An
adult locust can eat its own body weight every day, about 2.5 g. Adults that can fly are initially
sexually immature, but eventually become sexually mature and can copulate and lay eggs.
Solitary individuals always remain somewhere in the desert, ready to mate when conditions are
favourable.
Eggs
Laying
Eggs are usually laid in areas of bare sandy soil and require previous rainfall. Generally, the
female will not lay unless the soil is moist at about 5–10 cm below the surface. In soft sandy soils,
females have been known to lay when moisture is found only at depths below 12 cm. Before
laying, the female will often probe the soil, inserting the tip of her abdomen to determine if there
is enough moisture.
The female lays eggs in batches called pods. The eggs look like rice grains and are arranged
like a miniature hand of bananas. The pods contain fewer than 80 eggs in the gregarious phase
and typically between 90 and 160 in the solitarious phase. Swarms often lay egg pods in dense
groups, with tens and even hundreds of pods per square metre. Laying occurs in only a small
number of the apparently suitable sites. This behaviour, as well as an agent added to the egg pod
foam when adult females are crowded, will help induce gregarization of the next generation.
The number of egg pods a female lays depends on how long it takes for her to develop a pod and
how long she lives. An average of two pods per female is the norm. Because of natural mortality,
not all the eggs hatch and, of those that do, not all reach the adult stage. In optimal temperature
and habitat conditions, a single female can produce up to 16–20 viable locusts in a single
generation.
Development and incubation
The Desert Locust nearly always lays her eggs in soil that is wet enough to allow the eggs to
absorb sufficient moisture to complete their development. If eggs are laid in dry soil, they
desiccate (dry out) unless rain falls soon afterwards. The rate of development is exclusively a
function of the soil temperature at pod depth (Figure 3). There is a reasonably good relationship
between soil temperature and screen (air) temperature so rates of egg development can be
predicted satisfactorily from air temperatures and even from long-term mean values since
temperatures do not vary greatly between years for a given place and time of year in most of the4 WEATHER AND DESERT LOCUSTS
breeding areas. However, there can be exceptions to this, notably during the winter, when the
weather may be unusually warm, allowing development to continue.
Mortality
The proportion of eggs that survive to hatching varies widely according to habitat conditions
and the presence of egg parasites and predators. While eggs can dry up, especially if exposed
by wind, and can also be destroyed by persistent flooding, such events are uncommon. High
mortality may occur if soil temperatures are above 35°C. Estimates of total losses vary from about
5% to 65%.
Hoppers
Hoppers immediately moult until the first instar. They then pass through five instars (sometimes
six in the solitary phase), shedding a skin (moulting) between each. The development from eggs
into hoppers (wingless larvae or nymphs) is a function of temperature. The hopper development
period decreases with increasing daily air temperature from 24°C to 32°C (Figure 4). The
correlation with air temperature is less clear than with eggs because the hoppers can control
their body temperature to a considerable extent by basking or seeking shade.
When solitarious hoppers increase in number, their behaviour changes, they become
concentrated and can form groups. Grouping often occurs in open, less uniform, habitats, where
there are patches of relatively dense vegetation separated by large areas of bare soil.
Bands
As hoppers continue to concentrate, they become more gregarious and the groups fuse to form
bands. During warm and sunny days, hopper bands follow a pattern of behaviour alternating
between roosting and marching throughout the day. On overcast days, bands usually do not
move very far. For example, measurements for predominantly fourth instar bands range from
about 200 m to 1 700 m in a day. If the vegetation is very dry, bands may continue moving at
night in search of green vegetation. The band usually maintains a constant direction during a
Egg incubation period (days)
30
Egg development
26
22
18
14
10
25 26 27 28 29 30 31 32 33 34 35 36 37
Mean soil temperature (°C)
Figure 3. Relationship between egg development and temperature.
Eggs will hatch sooner under warmer conditions.WEATHER AND DESERT LOCUSTS 5
day; even a major obstruction is not always sufficient to change its path. The heading is often,
but not always, downwind.
Adults
After the final moult the new adult has soft wings that must dry and harden before it can fly; this
can take up to 10 days. Once able to fly, solitarious adults migrate at night when the temperature
is above 20–22°C and the wind is less than 7 m/s (13.6 knots). They usually take off about
20 minutes after sunset and can fly for up to 10 hours, usually flying for only a few hours at a time.
Individuals have been detected by radar up to heights of 1 800 m.
Swarms
The first swarms form several kilometres downwind from the main laying area and spend the
night roosting in vegetation. At sunrise, they descend to the ground and warm up by basking in
the sun. By mid-morning, swarms take off and will often continue flying until just before sunset
when they land and feed. If the weather is unusually hot, swarms may settle at midday before
flying off again in the afternoon. Swarms can occur as low-flying sheets (stratiform) or may pile
high in the air (cumuliform), with the top level as much as 1 500 m above the ground. Stratiform
swarms are flat, usually tens of metres deep, and often occur during cool, overcast weather
or in the late afternoon. Cumuliform swarms are associated with convective updrafts on hot
afternoons, especially common during the warmer and drier months of the year.
Like aircraft, locusts land and take off into wind. By mid-morning – or earlier, if the temperature
is warm enough for sustained flight – the entire swarm takes to the air. Sustained flight is rare if
50
Hopper development period (days)
Hopper development
40
30
20
24 26 28 30 32
Mean daily air temperature (°C)
Figure 4. Relationship between hopper development and air temperature. The warmer the
temperature, the faster hoppers will mature and become adults.6 WEATHER AND DESERT LOCUSTS
temperatures are below about 20°C. This lower limiting temperature is higher under overcast
conditions (about 23°C).
Swarms may fly up to nine or 10 hours in a day, moving downwind, although mature swarms
may sometimes move a short distance upwind if the wind is light. Swarms may be towed along
by winds aloft or they may be held back by winds near the surface which are usually slower
and often blow from a different direction. Although locusts within a swarm may be oriented in
different directions, the overall result is a downwind displacement. A swarm is usually displaced
at slightly less than the wind speed and may easily move 100 km or more in a day. It is not clear
with cumuliform swarms which wind level determines displacement. Swarms start to settle
about an hour before sunset as convection dies away. Very high airborne densities can occur
during this period. With many swarms, a considerable proportion of the locusts spend some of
the time on the ground, so the swarm nearly always moves at less than the wind speed. In the
absence of wind, locusts fly at about 3–4 m/s (5.8–7.8 knots).
Downwind displacement tends to bring locusts into an area during the season when rain is most
likely, for example, the Sahel of West Africa and the Sudan in the summer and the Red Sea coast
in the winter. Once rain falls, locusts will mature and breed. By the time the new generation
of adults is capable of sustained flight, the seasonal wind pattern may well have changed and
breeding conditions become poor. The locusts will then migrate rapidly, often over very great
distances, to another area.
Movements often take place during periods of particular winds, rather than coinciding with
the prevailing windflow. Locust adults and swarms do not always fly with the prevailing winds
but instead wait for specific types of winds to occur. For example, in West Africa in autumn, the
prevailing winds are from the north. Swarms will not move south with these winds. Instead,
they move northwards across the Sahara during the few days in which there are southerly winds
associated with an atmospheric depression (generally low-pressure systems, indicated by an “L”
on an air-pressure map) over the western Mediterranean. This is because the southerly winds are
warmer than the northerly ones.
Limit of invasion area
Recession area
Limit of invasion area winter/spring breeding areas
and resulting migration
The designations employed and the presentation of
material in the map(s) do not imply the expression of any summer breeding areas
opinion whatsoever on the part of FAO concerning the and resulting migration
legal or constitutional status of any country, territory or
sea area, or concerning the delimitation of frontiers.
Figure 5. Desert Locust recession area. The Desert Locust recession area covers about
16 million km2 from West Africa to western India. The invasion area extends to the north, south
and east of the recession area, covering some 30 million km2 – approximately four times the size
of the USA.WEATHER AND DESERT LOCUSTS 7
Similarly, adults and swarms leave the summer breeding areas in the interior of Sudan in the
autumn and move north-east towards to the Red Sea coast. In order to achieve this, they wait for
the prevailing north-easterly winds to be interrupted by south-westerly winds, which may be
warmer and more humid. In order for swarms to migrate from the interior of Arabia to central
Sudan at the beginning of summer, locusts in the Red Sea area can fly only on the rare days with
cross-sea upper-level winds, and even then the swarms appear to select a particular height at
which to fly.
Swarm densities vary considerably. The generally accepted figure for an average medium-density
settled (resting on the ground) swarm is about 50 million locusts/km2 (50 locusts/m2) across a
range varying from 20 million km2 to 150 million/km2. Swarms generally spread out when flying,
typically covering between two and three times the area they occupy when roosting in the sun
or feeding. Volume densities of flying swarms can be as high as 10 locusts per m3. About half the
swarms exceed 50 km2 in size.
Affected areas
During calm periods, Desert Locust infestations are usually present somewhere within about
16 million km2 of desert in 25 countries between West Africa and India (Figure 5). During
plagues, the number of countries and the size of the potentially affected area doubles,
representing about 20% of the Earth’s land mass. Within the recession area, that is, the normal
area occupied during calm periods, locusts move with the winds. These bring them into
particular zones during the summer (the Sahel and the Indo-Pakistan desert) and during the
winter/spring (North-West Africa, along the Red Sea, in Baluchistan (Pakistan) and the Islamic
Republic of Iran). If heavy rain falls in successive seasonal breeding areas, the locusts will
gregarize and, unless prevented by control, drought, or migration to unsuitable habitats, plagues
can develop. Rainfall over 25 mm in two consecutive months is usually assumed to be enough for
locust breeding and development.
50
45 Recession
Plague onset
40 Plague peak
Plague decline
35
Territories reporting swarms
Inferred total
30
25
20
15
10
5
0
1860 70 80 90 1900 10 20 30 40 50 60 70 80 90 2000 2010
Years
Figure 6. Desert Locust plagues in the past. There have been numerous plagues (orange bars)
in the past with calm periods (recessions) in between (grey bars). Since the 1960s, the frequency
and duration of Desert Locust plagues have declined, probably due to preventive control.8 WEATHER AND DESERT LOCUSTS
Individual locusts are not a threat to humans and crops. Only after gregarization and the
formation of bands and swarms are locusts a serious threat to food security of the human
population.
Locust migration
If the answer to all the following questions is “yes”, there is a good possibility that the adults or
swarms will migrate:
(a) Can the locusts fly?
(b) Is the temperature warm enough?
(c) Is the wind not very strong?
(d) Are ecological conditions dry where the locusts are now?
Plagues and upsurges
Desert Locust plagues have been reported since Phaoronic times in ancient Egypt. There is no
evidence that they occur after a specific number of years (Figure 6). During the last century,
plagues occurred in 1926–1934, 1940–1948, 1949–1963, 1967–1969, 1986–1989 and 2003–2005
(Table 1). Recent major upsurges were reported in 1992–1994, 1996–1998 and 2003. Locust
outbreaks can develop suddenly and unexpectedly in remote, inaccessible areas or in the
absence of regular surveys and incomplete data. Recent developments in satellite techniques to
monitor rainfall and vegetation have made it easier to detect potential areas of significant locust
activity that may require survey and control.
Table 1. Historical Desert Locust recessions, declines, upsurges and plagues
Recessions
Upsurges
Declines
Plagues
--- --- 1861–1867 ---
1868 --- 1869–1881 ---
1882–1888 --- 1889–1910 ---
1911 1912 1912–1919 1917–1919
1920–1925 1925–1926 1926–1934 1932–1934
1935–1939 1940–1941 1940–1948 1946–1948
1948 1949–1950 1949–1963 1961–1963
1964–1967 1967–1968 1968 1969
1969–1972 1972–1974 --- ---
1975–1976 1977–1980 --- ---
1981–1985 1985 1986–1988 1988–1989
1990–1992 1992–1994 --- ---
1995 1996–1998 --- ---
1999–2002 2003 2003–2005 2005
2006–
Source: Updated from Symmons and Cressman (2001):
FAO Desert Locust Guidelines, Chapter 1 – Biology and behaviour, p. 37.WEATHER AND DESERT LOCUSTS 9
WEATHER
Weather and Desert Locust biology
All the different phases in the life cycle of a locust require ideal meteorological conditions for
it to develop and cause the widespread damage that is often associated with locust plagues.
Meteorological data are important for both assessing the current locust situation and forecasting
its development (Table 2). Data, such as temperature, pressure and wind, are usually available
from NMHSs and should be used.
Information on meteorological and ecological parameters such as rainfall, soil moisture, soil
and air temperatures, surface and boundary winds, synoptic-scale patterns and the convective
state of the atmosphere are needed to understand and forecast swarm movement and the
various developmental stages. These stages include egg-laying, egg development, hopper
development, moulting, hardening of the wings, adult maturity, rate of movement of hopper
bands and adult swarms, and transition from the solitarious phase to the gregarious phase.
Rainfall data can be used to identify which areas may become suitable for breeding or where
green vegetation and locusts may be present. Temperature data can be used for estimating
the development rate of eggs and hoppers, as well as indicating whether it is warm enough for
adults to take off and fly. Wind and large-scale (synoptic) data are useful during periods when
adults or swarms are likely to be migrating or to estimate if there is an invasion threat from a
neighbouring country.
During recessions, the most important variables to monitor are rainfall, vegetation and soil
moisture. During outbreaks, upsurges and plagues, more environmental conditions play a role
(Table 2). Access to information on rainfall, vegetation quantity, soil moisture, temperature and
wind direction can be valuable in making accurate predictions and is essential for assessing the
potential for locust movement and the planning of control operations.
Rainfall
Rainfall data consist of rainfall location, date and amount to date. Because of the sparse coverage
of the measurement network and the variable nature of rainfall, such data can be inaccurate or
Table 2. Useful meteorological information in detection/prediction of outbreaks, upsurges
and plagues and in planning control operations during different stages of Desert Locust
development
Data Actual Forecast Use Stage
+1 day
Daily Breeding Outbreaks
+10 day
Rainfall Total Dekadal Migration Plagues
+30 day
Monthly Recessions
Seasonal
+1 day
Daily Upsurges
+10 day Maturation
Temperature Min/max Dekadal Plagues
+30 day Migration
Monthly Outbreaks
Seasonal
Direction
Wind Speed 12 h Migration Plagues
Height10 WEATHER AND DESERT LOCUSTS missing altogether. A rough estimate can often be obtained by asking locals during a survey. Rainfall estimates can also be derived from satellite observations. The date and quantity of the first rains of the season are useful. The occurrence of the last rainfall can sometimes be estimated by observing to what depth the soil is moist. Sometimes it may not be possible to find out the exact date or amount of rain, but a rough indication can still be useful. When relying on local surveys, it is important to remember that different people have different concepts of rainfall quantity. Some may say heavy while others may say light for the same rainfall. In general, light rainfall is defined as up to 20 mm, moderate from 21 mm to 50 mm and heavy as more than 50 mm. Also, rainfall quantity (how much rain fell?) may be confused with intensity (how hard did it rain within a given period of time?). The latter has no predictive power, however. Predictions of rain, obtained from NMHSs, can be useful in estimating plague and individual locust development. Eggs require moist soil conditions after laying as they need to absorb moisture to complete their development. They can be destroyed by flooding if extreme rainfall occurs after the laying takes place. Hopper development from the first instar to fledging (the final moult from the wingless fifth or sixth instar to winged adult) indirectly requires rainy conditions, since the hoppers require edible vegetation for survival. Adults start to mature when they arrive in an area that received significant rains recently. After fledging, the hardening of the soft wings of the locust is stimulated by rainfall. Temperature Egg development in the female depends on air temperature. Temperatures below 15°C are unfavourable. The rate of development of the laid eggs is a function of the soil temperature at the depth where the eggs are laid. Under conditions of high temperatures, egg development is more rapid. Egg mortality may occur when soil temperatures are above 35°C. Hopper development is also a function of temperature. The hopper development period decreases with increasing daily air temperature from 24°C to 32°C. Band movement is stimulated by air temperature. On warm, sunny days, the bands march throughout the day while, on overcast days, they do not move very far. Exceptionally high night temperatures can also facilitate some movement. Adults take off in temperatures above 20°C–22°C and fly with the wind (i.e. downwind). The migration of solitary adults occurs at night, usually 20 minutes after sunset, when the air temperature is above 20°C–22°C and the wind is less than 7 m/s (13.6 knots). Sustained flights require warm temperatures. Under temperatures < 20°C, sustained flights are rare. Swarms usually take off about 2–3 hours after sunrise. In sunny conditions, they can take off in temperatures of at least 15°C–17°C. Under cloudy conditions, take-off occurs when temperatures reach 23°C–26°C. Under cooler conditions, take-off can be delayed to some 4–6 hours after sunrise. Locusts generally will not take off if winds are greater than 6–7 m/s (11.7–13.6 knots). Wind Wind is the main transportation mechanism of locusts and also concentrates them by convergence. In certain parts of the locust area in certain seasons, winds are regular in speed and direction. These areas and winds can be recognized by using local climatological knowledge and hence the spatial distribution of direction and speed of swarm movements. Air brought into strong frontal systems and circulation of cyclones from the surrounding countries may collect locusts from any scattered solitarious populations, as well as survivors from multiple swarming populations. The associated widespread and scattered rains may provide suitable breeding conditions for rapid multiplication of these immigrants, causing an unexpected outbreak if local locust teams are not informed and surveys not conducted.
WEATHER AND DESERT LOCUSTS 11
Eggs can dry up if exposed to wind.
Hopper band movement is usually downwind.
Adult migration occurs at night when the air temperature is above 20°C–22°C and the wind is
less than 7 m/s (13.6 knots). The direction of the flight is downwind. The take-off wind speed of
swarms is usually < 6 m/s (11.7 knots). Swarms land about an hour before sunset as convection
dies away.
Swarms move under the influence of large-scale weather patterns on a synoptic scale. The
structure of swarms depends on weather conditions, governed by convective winds and low-
pressure systems. Cool, overcast weather favours stratiform swarms, while convective updrafts
on hot afternoons promote cumuliform swarms. Thus, swarms are usually stratiform in the
morning and become cumuliform in the heat of the day, when convection takes place from the
hot ground.
Seasonal changes in mean windflow bring locusts into specific zones. At the beginning of
summer, the locusts move southwards from North-West Africa into the northern Sahel; in the
autumn, they move northwards again. However, locusts do favour warmer winds associated with
atmospheric depressions and, in such cases, the movement need not follow the prevailing winds
in a given season.
Furthermore, wind speed is used to estimate the size of warms flying overhead. This can be
done with a simple formula: time (s) x width (m) x wind speed (m/s) = size of swarm (m). This
estimation has to be used with caution as it may overestimate the swarm size, but it can provide
valuable information on the extent and severity of upsurges and plagues.
It is hard to estimate the direction of a swarm movement from observations made inside it. Even
when a swarm passes directly over an observer and the direction of the upper parts of the swarm
are recorded as it approaches and recedes, uncertainties remain of the position relative to the
rest of the swarm. This is true for both the observer and the swarm, since often the observer
cannot be sure that successive sightings are of the same swarm. A single ground observer can
rarely do more than establish the general sense of displacement of a travelling swarm.
Weather and locust control operations
For locust control, as well as swarm movement, it is important to know the weather conditions
and windfields because these affect the concentration of potential control targets and the
suitability of conditions to carry out effective spraying. In planning Desert Locust surveys, the
following principles should be borne in mind (WMO, 1991):
(a) Locust populations move downwind;
(b) The hotter the wind, the greater the distance travelled per day;
(c) Highly turbulent (and correspondingly hot) winds disperse populations (reduce their area
density);
(d) Downwind movement eventually brings locusts into zones of wind convergence, where they
accumulate;
(e) As opposed to steady wind conditions, where turbulence disperses populations, convergent
winds have been shown to concentrate populations at least to the order of 10 000-fold;
(f) Locust populations are trapped in zones of wind convergence and participate in the diurnal
and daily cycle of movement of such zones. In some places and seasons, these movements
are relatively small and the locust population is correspondingly relatively stationary;
(g) Waiting for locusts to concentrate and form high-density populations is the most important
strategy for efficient and economic control of locusts, so that the concentrating effect of zones
of convergence must be utilized in control techniques.
In addition to their influence on locust development and migration, weather conditions are also
important in locust control operations (Table 3).12 WEATHER AND DESERT LOCUSTS Desert Locust control relies on conventional chemical insecticides that work mainly by direct contact action (droplets land on the locusts) and sometimes by secondary contact action (locusts touch the droplets on the vegetation) or stomach action (locusts eat the sprayed vegetation). Insecticides are usually neurotoxic, i.e. they kill the locust by interfering with its nervous system. The applied pesticide should be evenly spread over the target swarm either by hand, from a vehicle or by aircraft. An even distribution is accomplished by adjusting the size of the droplets to the wind speed and the location of application to the wind direction. Spraying should take place under very specific meteorological conditions to ensure maximal effects on locust populations. The best time for spraying is usually in the morning between 8 a.m. and 11 a.m. and after 4 p.m. Effective spraying may be possible before 8 a.m. if the wind is strong enough. It may also be possible to spray effectively between 11 a.m. and 4 p.m. if it is either cloudy and relatively cool (less than about 30°C) or if there is a steady wind of more than 4 m/s (7.8 knots) that will tend to prevent convection. Wind must be present when spraying because it is needed to spread or drift the spray over the target area. If there is no wind, the operator could be contaminated because the spray is not being carried away from him/her. There should be a steady wind of at least 2 m/s (3.9 knots) measured at a height of 2 m (a distinct breeze felt on the face). Spraying at wind speeds above 10 m/s (19.4 knots) (recognizable because dust and leaves are blown around) should be avoided, since it is not easy to predict where the spray will be deposited. Spraying is done at right angles to the wind direction. If carried out in up- and downwind directions, the result will be a large overdose on a very narrow strip of the target area and, possibly, poisoning of the operator during downwind spraying. During application, wind direction should also be monitored since: (a) if the wind drops or becomes very strong (more than 10 m/s (19.4 knots)), spraying has to be stopped and can only continue when the right conditions occur again; and (b) if the wind direction changes by more than 45°, spraying should recommence from the new downwind edge, over the remaining unsprayed area. It can be more efficient or necessary to spray swarms while they are flying using aircraft, if available, rather than using ground vehicles. The target swarms may be sprayed while they are on the ground, around the roost site, or while they are in full flight. Both stratiform swarms (low-flying up to heights of 100 m) and cumuliform swarms (flying up to heights of 1 000 m or more) may be sprayed. The swarm density is usually greatest in areas of convective winds, thus the highest success rate of locust control is achieved when spraying under these conditions. The advantage of spraying flying swarms is that flying locusts collect droplets efficiently since they are moving quickly (about 3 m/s (5.8 knots)) and their wings are beating faster. Temperature differences between the hot ground and the air are the drivers for convection and wind. Convection occurs when the Sun rises high in the sky and heats up the ground. It usually occurs on hot afternoons but may also occur in the late morning, especially if there is little wind. Locusts should never be sprayed when there is strong convection, because spray droplets may be carried outside the target area. Rainfall predictions are needed to time the control operations, since rain may wash off the insecticide from the vegetation. Spraying should not take place if rain is falling or seems likely to fall soon. Weather and Desert Locust plagues and upsurges The majority of Desert Locust upsurges and plagues develop as a result of unusual meteorological conditions such as those associated with cyclones and other extreme weather events that lead to heavy rainfall, which, in turn, causes ecological conditions to become extremely favourable for locust breeding. Plague declines are often attributed to the combined effects of control operations and unfavourable environmental conditions.
WEATHER AND DESERT LOCUSTS 13
Table 3. Relevant conditions, control applications and example products
Stage Relevant conditions Control applications Example products
Laying when soil is moist 0 cm–15 cm (rainfall Planning survey and control Maps of estimated
> 25 mm/month for 2 months) operations 10-day precipitation and
soil moisture
Soil temperature range for egg viability
15°C–35oC
(10–65 days)
Identification of areas suitable
Egg development rate increases with for breeding
Egg
temperature
Air temperature range of 20°C–35oC for egg
and hopper development Estimation of rate of egg
development
Eggs die if flooded or exposed to wind or high
soil temperatures (>35oC)
Rain needed for annual vegetation for food Planning survey and control Maps of estimated
(24–95 days; average 36 days)
and shelter operations 10-day precipitation
Development period decreases as air Identification of areas of green 10-day dynamic
temperature increases from 24°C to 32°C. vegetation greenness and dryness
maps
Hopper
In the early morning and late afternoon, Estimation of rate of hopper
hoppers bask on plant tops or the ground; at development 10-day Normalized
midday, they take shelter inside plants. Difference Vegetation
Control operations against Index (NDVI) maps
Bands march on warm, sunny days; bands do gregarizing hopper groups
not move on overcast days. and bands
Band movement is usually downwind.
Adults mature from 3 weeks to 9 months (2–4 Planning survey and control Maps of estimated
months is average). operations 10-day precipitation
Mature rapidly in areas receiving recent Identification of areas of green Daily wind maps and
significant rains; mature slowly in low vegetation forecasts
(2.5–5 months)
temperatures or dry habitats.
Estimation of rate of adult 10-day dynamic
Adult
Take-off 20 minutes after sunset above development greenness and dryness
20°C–22°C and wind < 7 m/s (13.6 knots) maps
Estimation of displacement
Fly downwind during the night at heights up rate and direction 10-day NDVI maps
to 1 800 m (generally < 400 m) with ground
speed of 25–65 km/h for up to 10 hours Control operations against
(2-hour average) gregarizing adult groups
Sustained flights are rare < 20°C.
Bask to warm up in the sun from sunrise to Planning survey and control Maps of estimated
mid-morning. operations 10-day precipitation
Take off about 2–3 hours after sunrise in warm Identification of areas of green Daily wind maps and
weather (4–6 hours after sunrise in cool vegetation forecasts
weather) and wind < 6 m/s (11.7 knots).
Estimation of rate of adult 10-day dynamic
Swarm
Take off in sunny conditions at least development greenness and dryness
15°C–17°C; in cloudy conditions at maps
23°C–26°C. Fly downwind during the day at Estimation of displacement
heights up to 1 700 m with ground speed of rate and direction 10-day NDVI maps
1.5–16 km/h until 2 hours before sunset or Control operations against
0.5 hours after sunset. swarms
Will not take off in winds > 10 m/s
(19.4 knots).14 WEATHER AND DESERT LOCUSTS
Plague of 1986–1989
The last major Desert Locust plague occurred from 1986 to 1989 and affected 43 countries. It
arose from widespread heavy rains that fell in Western Sahara in the late summer of 1986. The
plague finally ended in 1989 because of control operations and unusual winds that blew swarms
across the Atlantic Ocean.
Upsurge of 2003–2005
Four local outbreaks developed simultaneously and independently in the autumn of 2003 in
north-west Mauritania, northern Mali, Niger and north-east Sudan as a result of good rainfall and
breeding during the summer. Two days of unusually heavy rains occurred in October 2003 from
Senegal to Morocco, during which some areas in Western Sahara received more than 100 mm of
rain compared to an annual precipitation of about 1 mm (Figure 7). Environmental conditions
remained favourable for the next six months and Desert Locusts increased rapidly. During the
summer of 2004, large numbers of swarms from North-West Africa invaded the Sahel in West
Africa and quickly moved into crops. By then, the threat of a locust plague emerged, creating one
of the most dangerous situations since 1989. As the year progressed, the swarms migrated over the
continent, causing devastation. In November 2004, they appeared in northern Egypt, Jordan and
Israel for the first time in 50 years. Lack of rain and cold temperatures in the winter breeding area of
North-West Africa in early 2005 slowed down the development of the locusts and allowed national
locust control teams to break the breeding and migration cycle. Ground and aerial operations in
more than 20 countries treated nearly 130 000 km² of locust infestations. It took two years and
more than US$ 400 million to bring the regional plague to an end.
Upsurge of 1996–1998
A regional upsurge affected countries along both sides of the Red Sea from June 1996 to the
summer of 1998. It developed as a result of a cyclone in June 1996 and heavy rains in November.
Locust infestations were primarily concentrated in Saudi Arabia and, to a lesser extent, Egypt,
Eritrea, Ethiopia, northern Somalia, Sudan and Yemen. Large-scale control operations treated
more than 700 000 ha and brought the upsurge to an end by the summer of 1998.
Figure 7. Heavy rains in Western Sahara. Two days of unusually heavy and widespread rains in
Western Sahara in 2003 allowed ecological conditions to be favourable for breeding for the next
six months, giving rise to a Desert Locust upsurge in West and North-West Africa.WEATHER AND DESERT LOCUSTS 15
Outbreaks of 2006–2015
Due to unusually heavy and/or widespread rains, Desert Locust outbreaks occur almost every
year in part of the recession area. Recent outbreaks took place in:
• Eritrea (December 2006–March 2007)
• Yemen (May–September 2007)
• Western Sahara (September 2008)
• Yemen (March–June 2009
• Northern Somalia (March–June 2009)
• Mauritania (October–December 2009)
• India/Pakistan (October–November 2010)
• Mauritania (October 2010–May 2011)
• Sudan (October 2010–May 2011)
• Libya/Algeria (January–May 2012)
• Sudan (September 2012–April 2013)
• Sudan/Eritrea/Yemen/Saudi Arabia (August 201–March 2014)
• Northern Somalia (January–March 2014)
• Sudan/Eritrea/Saudi Arabia (October 2014–March 2015)
• Mauritania/southern Morocco (November 2015–May 2016)
The unusually heavy rains encountered before an outbreak can cause severe flooding in the
normally arid desert. Within minutes, runoff can trickle down and fill up the normally dry wadis,
making them difficult to cross (Figure 8). Within hours, large areas of the desert can be under
water. Once the floodwaters recede, the moist sand will be covered by a rarely seen green carpet
of annual vegetation – the perfect habitat for Desert Locust breeding.
Importance of weather information and conditions for Desert Locust control
The behaviour of the Desert Locust is directly influenced by meteorological parameters, such
as rainfall, temperature and winds arising from convergence, monsoons, position of storms
and depressions and the fluctuations in the position of seasonal convergence zones such
as the Inter-Tropical Convergence Zone (ITCZ) and Red Sea Convergence Zone. Accurate
Figure 8. A ground team during a typical Desert Locust survey. Unusually heavy rains in 2007
made it increasingly difficult to move in the already remote and rough locust-breeding areas in
the interior of Yemen. Yet ground teams are essential in trying to determine the extent of the
current problem and in guiding aerial control operations.16 WEATHER AND DESERT LOCUSTS
meteorological information is crucial for understanding locust population dynamics, outbreaks,
upsurges and plagues and for the undertaking of survey and control operations (Table 4). A
basic understanding of meteorology is useful when trying to analyse weather data in order to
predict locust developments. In countries threatened by a locust invasion, it is important for
meteorologists to have some knowledge of locust behaviour and for locust officers to have a
basic understanding of the influence of weather on breeding and migration.
The NMHSs in locust-affected countries monitor and forecast meteorological elements such
as precipitation, temperature, humidity and wind speed and direction. These elements are
crucial to forecasting locust breeding, maturation, migration and survival. Climatology (long-
term climate information) is an important element in strategic planning during recessions and
in advance of control operations. Climatology can provide the long-term mean weather and
deviations – unusually heavy rainstorms, for example – since it indicates the weather most likely
to be encountered. For more immediate actions, such as control operations, observations of
actual weather and weather forecasts are needed. Ideally, three types of information should be
available, namely: climate, actual weather and forecasts.
Locust control services can make use of meteorological information for planning survey and
control operations and for forecasting locust breeding and migration, for example:
(a) Where breeding is likely to occur;
(b) When the next generation is likely to be flying;
(c) Where and when that generation is likely to reach areas at risk of invasion;
(d) Effects of weather on logistics of survey and control – the moving of staff and materials, as
well as ground and aerial control operations against hoppers and swarms.
In general, locust movements take place in the temporary spell of warm winds ahead of cold
fronts. These depressions firstly bring the winds which make the movement possible and,
secondly, the rain necessary for making conditions suitable for breeding. Locusts are blown from
areas of divergence towards areas of convergence, which can be related to the position of the
ITCZ. Rainey (1951) was the first to discover the marked association between the occurrence
of locust swarms and the ITCZ. While the ITCZ does not change dramatically from day to day,
meteorologists should study its movements on a weekly to 10-day basis in order to provide
assistance to national locust surveys and control teams in the area and for upsurge prevention.
The differences in how data are collected and reported at the national and international levels and
the sparse coverage of meteorological data can be the cause of inaccurate forecasts and false security.
Rainfall, for example, is interpolated between observing stations, giving the impression of precise
knowledge, while the variable nature of rain can cause great differences in its spatial distribution.
Experience has demonstrated that a major gap remains in the identification of clear and useful
guidelines on the exact nature of meteorological products that must be provided at regular intervals.
Specific meteorological parameters are needed by NLCCs, as expressed at regional training
workshops for locust-affected countries on meteorological information for locust monitoring
and control, based on the type, frequency, format and whether it is an invasion (I – outbreaks,
upsurges, or plague) or a recession (R) period (see Table 4).
Interpreting weather maps
While environmental conditions, especially rainfall, are important for locust development and
breeding, wind and other atmospheric disturbances are most important for flying swarms.
Locust swarm movements are influenced by large-scale weather patterns and smaller-scale
wind motions. Locust swarms flying in a given area will tend to accumulate along any line
of convergence in the windfield, atmospheric fronts separating warm and cold airmasses.
These lines of convergence, such as the ITCZ or the sea-breeze front, consequently restrict the
movement of a swarm. The movement of these front lines is usually accompanied by heavy rains,
while the winds blow in the direction of the fronts.You can also read
